Method of manufacturing a seamless steel tube

ABSTRACT

A seamless steel tube is manufactured from a high alloy steel containing at least 5% of Cr while effectively preventing mandrel bar withdrawal troubles after elongation rolling of a material being rolled. A seamless steel tube of a high alloy steel containing at least 5% of Cr is manufactured by subjecting a material being rolled  3  to elongation rolling using a retained mandrel mill  1  in which the speed of a mandrel bar  2  in the axial direction is maintained constant while the speed of movement Vb of the mandrel bar  2,  the speed Vi of the material being rolled  3  on the inlet side of the retained mandrel mill  1,  and the speed Ve of the material being rolled  3  on the exit side of the retained mandrel mill  1  satisfy the relationship:  0.15  ≦Vb/{Vi+Ve)/ 2}≦0.70  by controlling the speed of movement Vb of the mandrel bar  2.

TECHNICAL FIELD

This invention relates to a method of manufacturing a seamless steel tube or pipe (hereinafter merely referred to as tube) containing at least 5% of Cr (in this description, unless otherwise specified, “%” means “mass %”). Specifically, the present invention relates to a method of manufacturing a seamless steel tube which effectively prevents mandrel bar withdrawal troubles after elongation rolling of a material being rolled which is made of a high alloy steel containing at least 5% of Cr.

BACKGROUND ART

In the manufacture of a seamless steel tube by the Mannesmann mandrel mill technique, first, a raw material in the form of a round billet or a square billet is introduced into a rotary hearth furnace and heated therein to 1200-1260° C. The raw material is then pierced with a piercer to form a hollow mother tube. A mandrel bar is then inserted into the bore of the hollow mother tube until it extends beyond the bore, and the mother tube is subjected to elongation rolling using a mandrel mill normally including 5 to 8 stands while gripping the outer surface of the hollow mother tube with grooved rolls, thereby reducing the wall thickness of the mother tube to a predetermined value. After the mandrel bar is withdrawn from the mother tube having a reduced wall thickness, the mother tube is subjected to rolling for sizing to a predetermined outer diameter using a reducing mill to manufacture a seamless steel tube.

Mandrel mills for elongation rolling are classified into full floating mandrel mills which do not constrain the movement of the mandrel bar in the axial direction, and retained mandrel mills in which the speed of movement (retained speed) of the mandrel bar in the axial direction is maintained constant by holding the rear end of the mandrel bar with a bar retainer installed on the inlet side of the mandrel mill. When using a full floating mandrel mill, the speed of movement the mandrel bar unavoidably varies, which tends to result in variations in the dimensions of the resulting mother tube. Therefore, in recent years, retained mandrel mills have been widely used in elongation rolling.

Patent Document 1 discloses an invention in which elongation rolling is performed using a retained mandrel mill for which the speed of movement of the mandrel bar is set at a value of at least 0.25 times the rolling speed at the entrance of each roll stand and at most 1.5 times the rolling speed at the exit thereof, thereby improving the quality of the inner surface of the material being rolled.

Patent Document 1: JP H08-294711 A1

DISCLOSURE OF INVENTION

The present inventors found that if a hollow mother tube of a high alloy steel containing at least 5% of Cr is subjected to elongation rolling using a retained mandrel mill based on the invention disclosed in Patent Document 1, a phenomenon in which it becomes difficult to withdraw the mandrel bar from the material being rolled after the completion of elongation rolling (in this description, this phenomenon will be referred to as a “mandrel bar withdrawal trouble”) occurs.

Mandrel bar withdrawal troubles are a cause of a marked decrease in productivity of elongation rolling using a retained mandrel mill. Therefore, it is an extremely important technical problem which must be solved in order to mass produce seamless steel tube of a high alloy steel containing at least 5% of Cr on an industrial scale.

The present invention is a method of manufacturing a seamless steel tube characterized in that a seamless steel tube of a high alloy steel containing at least 5% of Cr is manufactured by a process comprising subjecting a material being rolled to elongation rolling using a retained mandrel mill having a constant speed of movement of a mandrel bar in the axial direction in such a manner that the speed Vb of the mandrel bar, the speed Vi of the material being rolled on the inlet side of the retained mandrel mill, and the speed Ve of the material being rolled on the exit side of the retained mandrel mill satisfy the relationship: 0.15≦Vb/{(Vi+Ve)/2}≦0.70.

In the manufacturing method of a seamless steel tube according to the present invention, the above-described relationship is preferably satisfied by controlling the value of at least one of the speed Vb of the mandrel bar, the speed Vi of the material being rolled on the inlet side of the retained mandrel mill, and the speed Ve of the material being rolled on the exit side of the retained mandrel mill.

In accordance with the present invention, mandrel bar withdrawal troubles after elongation rolling of a material being rolled made of a high alloy steel containing at least 5% of Cr are effectively prevented, and the service life of a mandrel bar is greatly lengthened. Thus, according to the present invention, seamless steel tube of a high alloy steel containing at least 5% of Cr can be reliably mass produced on an industrial scale.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1(a)-1(d) are explanatory views showing the behavior over time of a mandrel bar and a material being rolled in a retained mandrel mill having 5 stands.

FIG. 2 is a graph showing the relationship between the rate of overlap K with the rate of occurrence of rolling troubles {(number of occurrences of rolling troubles/number of members being rolled)×100} or with the surface roughness Ra of a mandrel bar after it has been used for rolling of 50 members when a seamless steel tube was manufactured by carrying out elongation rolling of a material being rolled made of 13% Cr steel while varying the rate of overlap K in the range of 0.1 to 0.8.

LIST OF REFERENCE NUMERALS

1: retained mandrel mill

2: mandrel bar

2 a: rear end

3: material being rolled

BEST MODE FOR CARRYING OUT THE INVENTION

The best mode for carrying out a method of manufacturing a seamless steel tube according to the present invention will be explained in detail while referring to the accompanying drawings.

In this embodiment, first, a raw material in the form of a round billet or a square billet made of a high alloy steel containing at least 5% of Cr is introduced into a rotary hearth furnace and heated therein to 1200-1260° C. The raw material is then removed from the heating furnace, and it is subjected to piercing with a piercer to form a hollow mother tube.

The formation of the hollow mother tube can be performed in a conventional manner. Such a method is known to those skilled in the art, and a further explanation of this step will be omitted.

Next, a mandrel bar is inserted into the bore of the hollow mother tube until it extends beyond the bore, and the wall thickness of the hollow mother tube is reduced to a predetermined value by subjecting the hollow mother tube to elongation rolling using a retained mandrel mill normally comprising 5 to 8 stands (5 stands in this embodiment) while gripping the outer surface of the hollow mother tube with grooved rolls in each stand.

A bar retainer (not shown) for holding the rear end of the mandrel bar is installed on the inlet side of the retained mandrel mill for elongation rolling. During elongation rolling, the rear end of the mandrel bar is held by this bar retainer. As a result, the speed of movement Vb of the mandrel bar in the axial direction is maintained constant during elongation rolling.

In this embodiment, elongation rolling is carried out on a material being rolled such that the speed of movement Vb of the mandrel bar, the speed Vi of the material being rolled on the inlet side of the retained mandrel mill, and the speed Ve of the material being rolled on the exit side of the retained mandrel mill during elongation rolling satisfy the following relationship: 0.15≦Vb/{(Vi+Ve)/2}≦0.70. The reason why elongation rolling is carried out while satisfying this relationship will be explained below.

The present inventors found that if the retained speed of movement Vb of the mandrel bar, the speed Vi of the material being rolled on the inlet side of the retained mandrel mill, and the speed Ve of the material being rolled on the exit side of the retained mandrel mill satisfy certain conditions during elongation rolling, the length of the region of the material being rolled after the completion of elongation rolling (the material being rolled at this stage also being referred to as a “shell” in this description) contacting the mandrel bar (this region referred to in this description as the “overlapping region”) can be set at a suitable dimension, thereby making it possible to effectively prevent mandrel bar withdrawal troubles and minimize a decrease in the service life of the mandrel bar.

Namely, in the overlapping region of the material being rolled, a greater decrease in the temperature occurs due to extraction of heat through the contacting mandrel bar. As a result, the overlapping region undergoes thermal contraction so that its outer diameter decreases. This causes mandrel bar withdrawal troubles.

A procedure of calculating the ratio of the length of the overlapping region to the length of the shell (referred to in this description as the “rate of overlap”) will be explained.

FIGS. 1(a)-1(d) are explanatory views showing the behavior over time of a mandrel bar 2 and a material being rolled 3 in a retained mandrel mill 1 which includes 5 stands (1std-5std). In order to simplify FIG. 1 and for ease of explanation, the bar retainer which is provided for holding the rear end 2 a of the mandrel bar 2 has been omitted from FIG. 1.

As shown in FIG. 1(a), the material being rolled 3 prior to elongation rolling using the retained mandrel mill 1 (the material being rolled at this stage also referred to below as the “mother tube”) has a length Si. As shown in FIG. 1(d), the material being rolled 3 after elongation rolling using the retained mandrel mill 1 (the shell) has a length Se. In FIGS. 1(a)-1(d), the rate of elongation achieved by the retained mandrel mill is EL (EL=Se/Si), the speed of the material being rolled 3 on the inlet side of the retained mandrel mill 1 (the speed of the mother pipe) is Vi, the speed of the material being rolled 3 on the exit side of the retained mandrel mill 1 (the speed of the shell) is Ve (Ve=Vi×EL), and the speed of movement of the mandrel bar 2 is Vb.

The average rolling speed of the retained mandrel mill 1 in the time from the state shown in FIG. 1(c) in which the leading front end of the material being rolled 3 has just been gripped by the grooved rolls of the fifth stand 5std to the state shown in FIG. 1(d) in which the rear end of the material being rolled 3 has passed through the grooved rolls of the fifth stand 5std is (Vi+Ve)/2. In addition, the length of rolling time from the state shown in FIG. 1(c) to the state shown in FIG. 1(d) is Se/{(Vi+Ve)/2}.

Accordingly, the length ΔL by which the mandrel bar 2 advances during the time of elongation rolling from the state shown in FIG. 1(c) to the state shown in FIG. 1(d) is expressed by ΔL=Vb×Se/{(Vi+Ve)/2}.

In FIG. 1(c), if the length by which the mandrel bar 2 projects beyond the front end of the material being rolled 3 (the projecting length) is L5, then the length of the overlapping region L in FIG. 1(d) is expressed by L=L5+ΔL. If that projecting length L5 is empirically approximated as L5=0, then L=ΔL=Vb×Se/{(Vi+Ve)/2}. Accordingly, the rate of overlap K is expressed by the following Equation 2. $\begin{matrix} \begin{matrix} {K = {{L/{Se}} \approx {{{{Vb} \times {Se}}/\left\{ {\left( {{Vi} + {Ve}} \right)/2} \right\}}/{Se}}}} \\ {= {{Vb}/\left\{ {\left( {{Vi} + {Ve}} \right)/2} \right\}}} \end{matrix} & (2) \end{matrix}$

The present inventors found that if the rate of overlap K given by the approximate equation K=Vb/{(Vi+Ve)/2} becomes large, the temperature in the overlapping region decreases at an early stage, and to this extent, withdrawal troubles of the mandrel bar 2 more easily occur, while if the rate of overlap K becomes small, an early decrease in temperature of the overlapping region is suppressed and withdrawal troubles of the mandrel bar 2 are prevented.

However, if the rate of overlap K is too small, the mandrel bar 2 tends to suffer from wear, surface roughening, or even cracking or galling to the shell 3, and the service life of the mandrel bar 2 decreases.

Namely, if the rate of overlap K is too small, elongation rolling takes place in a narrow region of the mandrel bar 2, and the amount of work per unit length (or unit area) of the mandrel bar 2 locally increases. Therefore, wear, surface roughening, or the like of the mandrel bar 2 easily takes place.

As stated below, the rate of overlap K is inversely proportional to the average rolling speed in the retained mandrel mill 1. Therefore, if the rate of overlap K becomes too small due to making the difference in speed between the average rolling speed and the speed of movement of the mandrel bar 2 large, friction between the mandrel bar 2 and the material being rolled 3 becomes large. As a result, it becomes easy for wear, surface roughening, and similar problems to occur in the mandrel bar 2.

Thus, regarding the rate of overlap K, there exist a preferred upper limit in order to achieve ease of withdrawal of the mandrel bar 2 and a preferred lower limit in order to suppress a decrease in the service life of the mandrel bar 2.

FIG. 2 is a graph showing the relationship between the rate of overlap K and the rate of occurrence of rolling troubles {=number of occurrences of mandrel bar withdrawal troubles/number of members being rolled)×100} or the surface roughness Ra of the mandrel bar after rolling 50 members being rolled using the same mandrel bar. In this description, “rolling troubles” means troubles in withdrawing a mandrel bar from a material being rolled.

As shown in the graph of FIG. 2, if the rate of overlap K becomes larger than 0.70, the ease of withdrawal of the mandrel bar worsens. On the other hand, if the rate of overlap K becomes smaller than 0.15, the surface roughness of the mandrel bar after elongation rolling increases and the service life of the mandrel bar decreases.

As shown in the graph of FIG. 2, when the rate of overlap K is at most 0.70, withdrawal troubles of the mandrel bar 2 occurring after elongation rolling of the material being rolled 3 made of a high alloy steel containing at least 5% of Cr can be effectively suppressed, and when the rate of overlap K is at least 0.15, a decrease in the service life of the mandrel bar 2 can be suppressed.

From the same standpoint, the upper limit on the rate of overlap K is preferably 0.60 and more preferably 0.50. The lower limit on the rate of overlap K is preferably 0.2 and more preferably 0.3.

The rate of overlap K can be made to be in the range of at least 0.15 to at most 0.70 by controlling the value of at least one of the speed of movement Vb of the mandrel bar 2 which is held at its rear end by the bar retainer of the retained mandrel mill 1, the speed Vi of the material being rolled 3 on the inlet side of the retained mandrel mill 1, and the speed Ve of the material being rolled 3 on the exit side of the retained mandrel mill 1. The simplest way to control the rate of overlap K to be in the range of at least 0.15 to at most 0.70 is to maintain the values of the speed Vi of the material being rolled 3, the rate of elongation, and the speed Ve of the material being rolled 3 constant and to suitably vary the speed of movement Vb of the mandrel bar 2, as described in Examples 1 and 2 given below.

In this embodiment, a mandrel bar 2 is withdrawn from a material being rolled 3 which has a decreased wall thickness by elongation rolling performed in this manner. According to this embodiment, since elongation rolling is carried out with the rate of overlap K made at least 0.15 to at most 0.70 by controlling the value of at least one of the speed of movement Vb of the mandrel bar 2, the speed Vi of the material being rolled 3, and the speed Ve of the material being rolled 3, withdrawal troubles of the mandrel bar 2 occurring after elongation rolling of the material being rolled 3 can be effectively suppressed, and the occurrence of damage to the mandrel bar can be suppressed, thereby achieving an extended service life of the mandrel bar.

In this embodiment, the material being rolled 3 from which the mandrel bar 2 has been withdrawn without occurrence of withdrawal troubles is then subjected to sizing into a predetermined outer diameter using a reducing mill. In this manner, according to this embodiment, a seamless steel tube of a high alloy steel containing at least 5% of Cr can be reliably mass produced on an industrial scale.

EXAMPLE 1

The present invention will be explained more specifically while referring to examples.

A mother tube measuring 350 mm in outer diameter, 27.55 mm in wall thickness, and 9849 mm in length and having a steel composition containing 13% Cr and 6% Ni was subjected to elongation rolling using a retained mandrel mill to obtain a shell having an outer diameter of 295 mm, a wall thickness of 12.55 mm, and a length of 24685 mm.

Table 1 shows the speed Vi (mm/second) of the mother tube on the inlet side of the retained mandrel mill 1 in the elongation rolling, the rate of elongation EL (EL=Se/Si) in the retained mandrel mill, the speed Ve (Ve=Vi×EL; mm/second) of the shell on the exit side of the retained mandrel mill, and the speed of movement Vb (mm/second) of the mandrel bar together with the rate of overlap K=Vb/{Vi+Ve)/2}, the ease of withdrawal of the mandrel bar, and the state of damage to the bar.

In this example, the speed Vi (mm/second) of the mother tube, the rate of elongation EL, and the speed Ve (mm/second) of the shell were set to constant values for each condition in Table 1, and the speed of movement Vb of the mandrel bar was varied in the range of 500-2600 mm/second. Those runs having a rate of overlap K of at least 0.15 to at most 0.70 were examples of this invention, and those outside of this range were comparative examples.

The “ease of withdrawal” in Table 1 is indicated by “O” when the rate of occurrence of rolling troubles was less than 0.1%, it is indicated by “Δ” when the rate of occurrence of rolling troubles was at least 0.1% to at most 1.0%, and it is indicated by “X” when the rate of occurrence of rolling troubles exceeded 1%.

The “bar damage” in Table 1 is indicated by “O” when the surface roughness Ra of the mandrel bar after elongation rolling of 50 members using the same mandrel bar was less than 3 μm, it is indicated by “Δ” when it was at least 3 μm to at most 7 μm, and it is indicated by X when it exceeded 7 μm. TABLE 1 Ease of Vb/((Vi + with- Bar Vi EL Ve Vb Ve)/2) drawal damage Remarks 2000 2.51 5020 2600 0.74 X ◯ Comparative Example 2000 2.51 5020 2400 0.68 Δ ◯ This invention 2000 2.51 5020 2000 0.57 ◯ ◯ This invention 2000 2.51 5020 1000 0.28 ◯ ◯ This invention 2000 2.51 5020 800 0.23 ◯ Δ This invention 2000 2.51 5020 500 0.14 ◯ X Comparative Example

As shown in Table 1, if the rate of overlap K is in the range of at least 0.15 to at most 0.70, good results were obtained with respect to both ease of withdrawal and bar damage whereas when the rate of overlap K did not satisfy this range, the results were unsatisfactory for either the ease of withdrawal or bar damage.

EXAMPLE 2

A mother tube measuring 337 mm in outer diameter, 41.03 mm in wall thickness, and 5473 mm in length and having a steel composition containing 18% Cr and 8% Ni was subjected to elongation rolling using a retained mandrel mill to obtain a shell with an outer diameter of 295 mm, a wall thickness of 31.03 mm, and a length of 8115 mm.

Table 2 shows the values of the same parameters as those set forth in above-described Table 1. In this example as well, the speed Vi (mm/second) of the mother tube, the rate of elongation EL, and the speed Ve of the shell (mm/second) were set to the constant values as shown in Table 2 for each condition, but in contrast to Example 1, the speed of movement Vb of the mandrel bar was varied in the range of 250-1400 mm. In the same manner as in Example 1, the test results are compiled in Table 2. TABLE 2 Ease of Vb/((Vi + with- Bar Vi EL Ve Vb Ve)/2) drawal damage Remarks 1500 1.48 2220 1400 0.75 X ◯ Comparative Example 1500 1.48 2220 1250 0.67 Δ ◯ This invention 1500 1.48 2220 1000 0.54 ◯ ◯ This invention 1500 1.48 2220 800 0.43 ◯ ◯ This invention 1500 1.48 2220 400 0.22 ◯ Δ This invention 1500 1.48 2220 250 0.13 ◯ X Comparative Example

As shown in Table 2, when the rate of overlap K is in the range of at least 0.15 to at most 0.70, good results are obtained with respect to the ease of withdrawal and bar damage, whereas if the rate of overlap does not satisfy this range, the results for either the ease of withdrawal or bar damage are unsatisfactory. 

1. A method of manufacturing a seamless steel tube characterized in that a seamless steel tube of a high alloy steel containing at least 5% of Cr is manufactured by a process comprising subjecting a material being rolled to elongation rolling using a retained mandrel mill having a constant speed of movement of a mandrel bar in the axial direction in such a manner that the speed (Vb) of the mandrel bar, the speed (Vi) of the material being rolled at the inlet side of the retained mandrel mill, and the speed (Ve) of the material being rolled on the exit side of the retained mandrel mill satisfy the following Equation
 1. 0.15≦Vb/{(Vi+Ve)/2}≦0.70  (1)
 2. A method of manufacturing a seamless steel tube as set forth in claim 1 wherein the above-described Equation 1 is satisfied by controlling the value of at least one of the speed (Vb) of the mandrel bar, the speed (Vi) of the material being rolled on the inlet side of the retained mandrel mill, and the speed (Ve) of the material being rolled on the exit side of the retained mandrel mill. 